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Chapter 24 Magnetic Fields

Chapter 24 Magnetic Fields. Objectives. 24.1 Describe the properties of magnets and the origin of magnetism in materials 24.1 Compare various magnetic fields. Objectives. 24.2 Relate magnetic induction to the direction of the force on a current carrying wire in a magnetic field

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Chapter 24 Magnetic Fields

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  1. Chapter 24 Magnetic Fields

  2. Objectives • 24.1Describe the properties of magnets and the origin of magnetism in materials • 24.1 Compare various magnetic fields

  3. Objectives • 24.2Relate magnetic induction to the direction of the force on a current carrying wire in a magnetic field • 24.2 Solve problems involving magnetic field strength and the forces on current-carrying wire, and on moving, charged particles in magnetic fields • 24.2 Describe the design and operation of an electric field motor

  4. Magnetic Poles • Magnets are polarized • One positive end, one negative end • Compasses are just magnets free to spin • If you slice a magnet in half, you retain two opposite poles • No scientist yet to make a monopole

  5. Opposites Attract

  6. The Earth • Earth is a big magnet. The actual location of the magnetic poles change every year by about 40 miles

  7. Magnetic Declination • Your compass points to the magnetic pole, not to the true north pole • As you approach the poles, you have to add/subtract degrees to go the right direction

  8. The north arrow on the compass rose (the large N) is pointed towards the place on the horizon directly beneath the North Star! That is, towards true north! • And the needle (of course) points towards magnetic north! So the magnetic declination for this locality is 45 degrees west

  9. Temporary Magnets • Most metal objects are NOT magnetic but they can become magnets when in contact with a magnet • A magnet holding a nail polarizes the nail to make it a temporary magnet

  10. Permanent Magnets • Of the common metals, only 3 actually produce a magnetic field • Iron, Nickel, Cobalt (The Iron Triad) • Many rare earth elements also do, but we don’t need to know about those

  11. Magnetic Flux • The strength of the magnet. • Strongest where the field lines are closest together • Flux per unit area is proportional to strength

  12. First Right Hand Rule • Current moving causes a field (the current is Positive, opposite of the actual electrons) • Bigger Current = Stronger field • Thumb points towards North

  13. Solenoid: Coils wrapped around with a current moving through

  14. Thumb points towards the North End

  15. How computers work is through magnetized domains

  16. Computers • Surface covered in magnetic particles • Recording: Current goes through disk drive’s read/write head (em magnet iron core). Current induces a magnetic field onto bits • Magnetic particles line up and orient • To read/retrieve, no current is sent to the read/write head. The bands (bits) induce a current to the read/write head. 0 or 1

  17. Questions • A long, straight, current carrying wire runs from North to South. • A compass needle placed above the wire points with its N-pole toward the east. In what direction is the current flowing? • What direction would the needle point if the compass were below the wire points?

  18. More on Fields • Current induces a field and causes a force to act on current carrying wire (or other wires) • Uses below • Loudspeakers (wire moves) • Galvonometer: Measures the current • TV’s: Lining up the electrons

  19. Magnetic Fields • Points towards the South Pole • So, South must be Negative end • Since magnetic fields point towards the negative charge • Earth’s North pole • Is the south magnet

  20. Magnetic Field • Measured in Teslas

  21. Calculating Strength of Magnetic Field • F = BIL • Force = (Magnetic Field Strength) (Current) (Length of wire) • A wire 0.50 m long carries a current of 8.0 Amps at a right angle to a 0.40 T magnetic field. How strong a force acts on on the wire?

  22. Force on a moving charged particle • F = Bqv • Force = Field x Charge x Velocity of particle • How strong a force acts on a moving electron which is traveling a 3,000,000 m/s through a uniform magnetic field of 0.04 T at right angles to the field.

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